1. Field of the Invention
This invention relates to the field of quantitative computed tomography and, in particular, to a system for reproducibly measuring the CT number of in vivo tissue samples.
2. Description of the Related Art
Computed Tomography (CT) was developed as a method for qualitatively analyzing soft tissues by non-invasively obtaining three-dimensional pictures of affected areas in a patient. Diagnosis of many pathological conditions, however, requires quantitative analysis of the affected organs or tissues. This has historically necessitated painful and often dangerous invasive methods of obtaining tissue samples for in vitro analysis, for example to determine the relative density or composition of the affected organs.
Because of the non-invasiveness of CT analysis and the ability to scan a precise three-dimensional cross section of the patient, it has been proposed to use the data resulting from a CT scan quantitatively as well as qualitatively.
One type of condition recognized as suitable for quantitative CT analysis is osteoporosis. Osteoporosis evaluation involves the measurement of bone mineral density in affected spinal tissues. However, quantitative CT analysis of osteoporosis has suffered from practical disadvantages which have severely limited its accuracy, despite the potential advantages obtained by such non-invasive quantitative CT analysis, and the widespread availability of CT scanners.
The primary problem in utilizing quantitative CT analysis has been calibration of the CT numbers obtained in a CT scan to account for patient variability and positioning of the patient. The attenuation of the X-ray spectrum used in a CT scan is affected not only by tissues in the region of interest, for example a particular vertebra, but also by the thickness and density of tissues around the region of interest which also attenuate the X-ray beam. Since the density of the affected bone tissues is itself variable, it is not possible to calibrate the CT scan without an additional reference.
Recently, attempts have been made to correct for the problem of CT number variability of a tissue situated in a heterogeneous surrounding of other tissues by providing a reference, known as a "phantom" , located in the path of the CT scan but external to the patient. Such reference "phantoms" have been partially successful in accounting for effects of X-ray beam "hardening" and scatter that are correlated with patient variability, but are nevertheless subject to such problems as local differences in X-ray spectra and in scatter distribution between the phantom and the target tissue, variations resulting from patient movement with respect to the reference phantom, and volume averaging difficulties resulting from the current practice of taking the mean of the CT distribution.
In order to overcome the problems associated with the use of external reference phantoms, it has been suggested that internal reference tissues located near the affected tissues could be utilized. Potential in vivo reference tissues include fat and muscle, whose relatively invariable densities and close proximity to vertebral bone tissues make them suitable candidates for use in calibrating CT scans without the need for external reference phantoms.
However, it has heretofore been impossible to reproducibly obtain meaningful CT numbers for in vivo fat and muscle tissues because no method has been available for eliminating the inevitable effects of fat and muscle tissue mixing, and to account for the nonuniform distribution of such tissues throughout the CT slice.
Any quantitative distribution obtained from a CT scan for areas of fat and muscle will invariably include areas representing fat and muscle mixtures, as well as background from other tissues in the path of the scan. Because the mixing is essentially random, CT numbers representing such mixtures are of no use in deriving a reference standard for correlating CT numbers with tissue density.